When using thermal flow meters, the accuracy of mass flow rate measurements of media traveling within a conduit can be adversely affected by random and unpredictable turbulence within the flowing media, as well as by the fact that flow rates are not uniform across the conduit cross section. Flow meters typically employ high performance sensing elements that can be uniquely designed for application requirements with precision signal processing and calibration of sensing elements and transmitting electronics. Random turbulence, or non-uniform flow, within the flowing media introduces inconsistencies in the transducer readings.
Thermal dispersion flow meters are well known and are commonly used to measure the flow of media within a conduit. Thermal technology utilizes the relationship between flow rate and cooling effect for direct measurement of mass flow rate. The media flowing in a conduit affects the temperature of sensing elements and this effect is used to create an electrical signal that can be processed to indicate the flow rate or mass flow rate of the media within the conduit.
Flow conditioning devices may be used to overcome random turbulence properties that occur in applications where non-ideal upstream flow conditions exist. These turbulence properties may be caused by valves, bends, or elbows, for example, within the conduit, as well as flow rates of the media, and viscosity properties of different types of media. The elements employed by thermal dispersion mass flow meters can suffer accuracy problems due to non-ideal flow conditions in the vicinity of the sensing elements. Such non-ideal flow conditions that exist upstream from the sensing elements can create inaccuracies in the readings obtained from the sensing elements.
Within the art of flow meters, numerous flow conditioning devices have been taught. Examples of known flow conditioners are those that use bars, perforated plates, tube bundles, or tab structures to condition media to enhance sensor readings.
One turbulence inducing prior art device is disclosed in U.S. Pat. No. 5,780,737. This sensor employs a bar mounted closely upstream from a transducer for the purpose of flow conditioning. The bar generates a predictable vortex stream (turbulence) a short distance upstream of a flow sensing element to counteract the random or unpredictable turbulence that exists within the media flowing through a conduit. The vortex stream generated by the flow conditioning bar is consistent and predictable compared with the non-conditioned turbulence within the flowing media upstream of the bar. Thus, any existing random turbulence within the flowing media is essentially overridden by the turbulence created by the vortex generating bar.
A completely different type of flow conditioner is shown in U.S. Pat. No. 4,929,088, which includes several radially, or longitudinally, or both, spaced tabs to create a mixing effect as well as conditioning the flow of the media in the conduit.
In order to measure media flowing within a conduit by means of a thermal flow meter, minimum straight runs of the conduit are typically needed for improved accuracy. In order to achieve optimum performance in industrial flow metering systems, upstream and downstream straight run requirements are typically quoted at about 20 conduit diameters upstream and about 10 diameters downstream. These straight run lengths are typically necessary in order to create a consistent flow profile and allow dissipation of the turbulence in the media that may result from elements such as bends, elbows, and valves in the conduits carrying the media. Implementing straight runs of these lengths is not always easy and sometimes impossible to satisfy in any particular installation. Metering systems with insufficient straight run lengths can suffer somewhat degraded meter accuracy if a consistent flow profile is not able to be developed.